1. Field of the Invention
This invention relates to optical fiber amplifiers, especially to a method and an apparatus for enhancing optical signal power to amplified spontaneous emission (ASE) power ratio in optical fiber amplifiers.
2. Background
In high bit-rate optical fiber transmission systems (channel rate >40 Gb/s), it is often required to use EDFA to amplify the individual channels power to compensate for large insertion loss in optical transponders. One of the important performance requirements is the signal power to total ASE noise power ratio (Ps/ASE) in such applications. Although the EDFA is used for single channel operation in this case, it has to provide optical gain over entire C-Band or L-band as the optical channel can be located at any ITU grid. As a result, the total ASE power over entire C-band or L-band can be quite significant compared with the amplified optical signal power. If the signal to ASE ratio is small, the optical receiver may be saturated, and the receiver's signal to noise ratio (SNR) can be degraded due to the spontaneous-to-spontaneous beat noise caused by large total ASE power from the EDFA. Taking a conventional C-band single channel EDFA as an example, the ASE bandwidth of the EDFA can be from 1528 nm to 1562 nm, total ASE can be comparable to the amplified signal power, i.e., the signal to ASE power ratio can be as low as 0 dB. This can cause significant bit error rate (BER) performance degradation in the receivers. Therefore, a means for enhancing Ps/ASE ratio becomes a necessity. In theory, one could use a tunable band pass filter for this purpose. However, tunable filter is generally very expensive and therefore it is cost effective. In this invention, a low cost solution that uses switchable band-pass filters in EDFA is disclosed.